Metallic and metalloid nitride, oxide, phosphide, sulfide, and arsenide films possess unique properties that make them useful coating materials for various applications. For example, nitride films can be used in applications where properties such as high temperature semiconduction, insulating thermal conduction, piezoelectric characteristics, thermal shock stability, and light emission capabilities are desired. Furthermore, nitride films can be used for device capping/protection, and in applications where extreme wear resistance for contacting surfaces is desired. Oxide films are often used to protect against corrosion and abrasion at high temperatures. Phosphide films can be used in applications that utilize the semiconduction properties they frequently display. Films containing mixed species, such as mixed oxide-nitride films, can also be used to protect against corrosion and abrasion at high temperatures.
Various techniques are used for the deposition of films such as these, including sputtering, plasma, and chemical vapor deposition ("CVD") techniques. For energy efficiency, convenience, and protection of the substrate material, it is desirable to carry out the deposition of films at as low a temperature as possible. Thus, conventional chemical vapor deposition techniques are the most desirable because they do not utilize such energy intensive physical processes as plasma enhancement and sputtering.
Chemical vapor deposition is a process "wherein a stable solid reaction product nucleates and grows on a substrate in an environment where a vapor phase chemical dissociation or chemical reaction occurs." M. G. Hocking et al., Metallic and Ceramic Coatings, Longman Scientific and Technical: Essex, England, 1989, Chapter 4. In CVD, a heat-decomposable volatile precursor, such as an organometallic compound, is contacted with a substrate. The substrate is heated to a temperature required for decomposition of the precursor to the compound desired (typically, 200.degree.-1500.degree. C.). A coating thus forms on the substrate. Herein, such a decomposable volatile compound is referred to as a molecular organic chemical vapor deposition ("MOCVD") agent or precursor.
In addition to the low energy requirements of CVD techniques relative to plasma and sputtering techniques, CVD techniques are advantageous for forming films on substrates having an uneven surface or projections. Furthermore, with CVD techniques the compositions of the films can be more readily controlled. Also, films can be more readily prepared without contamination of, or damage to, the substrate using CVD techniques.
In the electronics industry, as well as other industries, there is a growing need for volatile precursors of various metals to be used in the chemical vapor deposition of metal nitride films, metal oxide films, and the like. An important characteristic of such metal precursors is the capability of evaporation or sublimation to give a metal-containing vapor or gas that can be decomposed in a controlled manner to deposit a film onto a target substrate. Relatively few organometallic complexes, however, are sufficiently volatile for use as MOCVD precursors.
Traditionally, metal nitride films have been deposited with CVD techniques using a separate molecular source for each of the elements in the product. Generally, the films consist of binary products, i.e., compounds containing only two types of elements. The components of the separate-source precursors typically include ligands such as hydrides, halides and alkyls. Metal alkyls and hydrides can be quite toxic and/or flammable, however. Furthermore, the presence of a halogen atom in a film can promote corrosion. Thus, a need exists for more desirable MOCVD precursors.
To avoid some of the problems associated with separate-source MOCVD agents, single-source MOCVD agents are more desirable. Single-source precursors are compounds in which the elements of the product are incorporated into one starting material. They are desirable, for example, because the stoichiometry of the reactants can be more readily controlled than it can be with separate-source precursors. Also, the incorporation of impurities, such as carbon, into the deposited film can be more readily avoided using single-source precursors.
Single-source precursors can include Lewis acid-base donor-acceptor adducts wherein the Lewis base and the Lewis acid each contain one of the required elements for the product. The donor-acceptor adduct bonds, however, are usually not as robust as normal covalent bonds. Thus, dissociation occurs relatively easily upon heating and before volatilization with these adducts. As a result, the use of excess Lewis base is often required to produce quality films. Other currently used single-source materials are not generally stable in air, volatile at reasonable temperatures, or capable of producing reproducible films without impurities. Thus, a need exists for single-source chemical vapor deposition precursors that overcome the problems associated with the prior art separate-source and single-source materials.